The present invention relates to a method of detecting an abrupt variation in a periodic electrical quantity wherein a criterion for the occurrence of the abrupt variation is formed from the samples of the periodic quantity. Such a method is used, for example, in power system protection technology to detect the occurrence of a short circuit in an electric power supply system, for example.
In a digital feeder protection system (7SA511) manufactured by Siemens AG, an abrupt variation in a periodic electrical quantity is detected by comparing two sampled values of the periodic quantity separated by one period of the periodic quantity. If the deviation is zero, there is no abrupt variation in the periodic quantity. If, however the deviation is not zero, there may be such a variation. According to this method, however, the zero value of the deviation is nevertheless uniform, i.e., it can be achieved without an abrupt variation only when the alternating quantity does not have any signals with frequencies that deviate from their nominal frequency. If this is the case, there is always the risk of a non-steady process being detected with the method, although there is no abrupt variation at all. To avoid faulty detection, a threshold value is set so high that there is no spurious response with the usual deviations in the periodic electrical quantity from the rated frequency. This in turn has the result that, the occurrence of abrupt variations in a periodic quantity can be detected only when there is a great variation.
Therefore, an object of the present invention is to propose a method of detecting an abrupt variation in a periodic electrical quantity with which such a variation can be detected reliably even when the abrupt variations are relatively small.
To achieve this object, the periodic quantity is weighted in an FIR filter with a transfer function with a double zero position at the nominal frequency not only itself, but also in its first derivation according to time, and the output signal of the FIR filter is monitored to determine whether it reaches a threshold value indicating the occurrence of an abrupt variation in the periodic quantity.
An advantage of the method according to the present invention is that, due to the use of an FIR filter with a double zero position at the nominal frequency, the FIR filter delivers an output signal with a much smaller value than the known method in the case of especially critical periodic quantities in the present context with a relatively minor deviation from the nominal frequency. Therefore, the achievable sensitivity is much greater with the method according to the present invention than with the conventional method.
In an advantageous embodiment of the method according to the present invention, an FIR filter with a transfer function having a zero position at a frequency value of zero is used. Therefore, to further increase the reliability of the method according to the present invention, effects of offset phenomena can be avoided by an analog-digital conversion to the measured result following the sampling.
With an advantageous circuit arrangement for carrying out the method according to the present invention, a comparator is connected to the output end of an FIR filter that receives the periodic electrical quantity at the input and delivers a signal characterizing the abrupt change in the periodic electrical quantity when an output signal of the FIR filter reaches the threshold level. However, it is also readily possible to implement both the FIR filter and the comparator in the form of a data processing system, and to have the method according to the present invention run by means of only the data processing system, if the latter also performs the sampling and the analog-digital conversion of the periodic electrical quantity to be monitored.